Mast step truss, inevitably:(

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Bob McGovern
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Re: Mast step truss, inevitably:(

Post by Bob McGovern »

Huh. For some reason I presumed you are an engineer; maybe because you do things (like casually tearing down a Yanmar) the meticulous way I wanted to do things when aiming to become one. :D Anyhow, as a clever person who understands forces & how to handle them, have you any improvements to suggest? Request open to everyone -- I'm mostly an English/Irish poetry specialist. :lol:

Someday, we'll just 3D-print new parts to fit our Ballads:

https://www.youtube.com/watch?v=hmxjLpu2BvY

(But not today. :P)
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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs »

One thought is that if the truss is buried in resin, like the original, there's no way to monitor corrosion. I know stainless doesn't like being sealed up ... what about aluminum? But I'll bet that whatever you finally do it'll outlast the boat!
Bob McGovern
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Joined: Fri Oct 26, 2012 3:08 am
Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern »

Most corrosion-resistant metals rely on exposure to fresh oxygen to generate & renew the thin, inert oxide layer that protects the fresh metal beneath. That's less true of titanium; more true of aluminum; and extremely true of austenitic stainless steel (series 300). Stainless chainplates usually fail where they pass thru the deck core. Presence of water + absence of oxygen = crevice corrosion. A stainless truss buried in resin would rot as fast as a mild steel truss, which is why good drainage & air circulation is preferred over encapsulation. Albin likely thought they were protecting the steel by pouring resin around it.

Aluminum is a bit less reliant on air exposure but more prone to galvanic corrosion. One reason I'm leaning toward aluminum for our truss is because stainless can 'eat' aluminum. So the truss might be fine, but the mast base may corrode. This is a fairly common problem with aluminum masts stepped onto iron keels. I wonder if keeping the metals closer in type would reduce the chance of creating a battery in your bilge?

Titanium would be really fine, tho a bit shock-prone & would cost more than the market value of our boats.:) Composites are attractive, tho they have strange properties sometimes & carbon fiber also eats aluminum. I bet G10 could be made into an excellent truss: it has a face compression strength around 65k psi, tho I have no idea what its edge strength is. And others here have made very good replacements out of wood, which is a superb material if you can keep it dry. As I mentioned to a fellow with a Ranger 37 who was needing to shim his mast foot, Westsail 32s often used a $3 paving brick under their masts. It is possible to overthink this, and that's a disease I have.;) (A sack of concrete mix poured into the cavity might be a pretty good solution! Sink some glue-in studs into it afterwards. Hmmmmm. You'd need to declare your extra ballast to the class measurer, tho.)

So, how do we allow for inspection of the truss cavity after the cabin sole is replaced?
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

Perhaps an access panel that wraps around the mast foot with lateral supports to prevent sag? Probably best if it were fastened down in some way to stop unsuspecting crew members from disappearing down the hole or tripping on their way to the heads... It would also likely become one of the lowest points in the bilge so would inevitably collect water, humm.

Just to throw the cat in amongst the pidgins, I asked our friendly local engineering plastics company what they would recommend as a steel substitute, and they suggested:

http://www.directplasticsonline.co.uk/Acetal_Sheet.html

http://www.directplastics.co.uk/pdf/dat ... 0Sheet.pdf

I fear it would be prohibitively expensive though....
Bob McGovern
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Re: Mast step truss, inevitably:(

Post by Bob McGovern »

My first instinct was toward composites -- they really do offer tremendous advantages in many ways, mostly their insensibility to water. And you can find I-beams in FRP that are adequate structurally.

http://www.eplastics.com/Plastic/Fiberg ... een-I-Beam

Minimum 6m order, tho. Carbon fiber problematic due to galvanic issues? Would be a good choice if any Ballad has a carbon mast!;)

I use a lot of phenolic countertop material that has acceptable edge strength and terrific face and beam strengths. Thought about building the truss from that. The problem with composites is knowing how good the finished article really is. I guess that's true of welded metals and some imported metals as well. But even at the highest levels of design and fabrication, composites have a tendency to fail unexpectedly, for reasons that are hard to analyze later. Consider the last Volvo Ocean race, when entire hulls were delaminating on the first leg, or the recent Gunboat catamaran that dropped its carbon rig and was abandoned fresh from the factory. Some plastics (like HDPE) begin yielding under loads much lower than their ultimate strength. Others like carbon fiber tend to fail explosively right at their yield strength.

At first, metals had the same learning curve composites are going thru now: boiler explosions, Liberty Ships, etc. These days, metals are such a known thing, made in large batches in a few factories to exacting standards. If you buy 316L Stainless from an ISO9001 factory, you pretty much know what you are getting. And steel especially is so forgiving. Our poor truss was nothing but rust flakes and lace, yet the mast was still up! OTOH, acetal wouldn't rust in the first place.... :D There's no one nearby who could make a reliable composite truss for me, and my own skills & knowledge are not good enuf.

Anybody building a truss from composite, please detail it here!
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

Interesting with regards to composite trusses Bob, most modern yachts sporting composite trusses must do so for some reason.

I finally had some time to take a look at my own truss, and it turns out for anyone who is needs to inspect theirs, there is no need to drill holes to inspect the truss with an endoscope! The bulkhead that is immediately aft of the truss has a half inch gap between it and the cabin sole meaning you can get an endoscope in there, or even your iPhone! I did happen to drill my inspection hole before I discovered this fact, no matter, some thickened epoxy will seal that one up. The fact that gap is there explains why so many trusses end up rotting - if a Ballad ever has had standing water that comes up to the cabin sole (such as rainwater over a period of time where a Ballad has been left), the truss will be underwater and fully exposed.

Here is a good picture of my truss, looking aft actually taken with my iPhone:

Image

I'm glad I've had a look, and frankly I'm pretty concerned about the big lump of rust supporting my mast. I had a good dig at it with a screwdriver and there was solid metal underneath, so I went ahead and filled the void with as much of the rust treatment as I dared. I'll do another coat before I seal the inspection hole up.

Image

Edit: Hard to make it out as the endoscope has pretty shady quality, however, you can see me probing with the screwdriver and some bits of rust flaking off. The front face looks okay from what I could see. I will probe some more and try and scrape off as much of the rust next time I visit her.

https://youtu.be/HYrNaqG-bm0
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

Ominous silence? :D Prudent to replace do we think? Any news on your fabrication Bob?
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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs »

It's "The elephant in the room" ... nobody wants to know! Thanks for sharing the info of the gap between the floor and the bulkhead. I'm going to scope mine next week and will post the results. Hopefully ... no elephant!
Bob McGovern
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Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern »

Ummm ... that truss looks ... um .. sail carefully this summer, eh? :shock: :lol: The worst material erosion is right where the steel meets the resin, where water sits against it. Probably you'll be good for the season -- thing's lasted forty-odd years -- but based on the visual evidence, truss replacement might move to the top of the queue next time your mast is out. And it wouldn't be that awful a job once you commit to it, esp. if you have the truss already made or a fabricator lined up. Two people could chop out the old truss, install the new, fix the sole, and be sailing again in three partial work-days. Our delay is entirely down to my laziness. :D A motivated Ballad owner with sailing in prospect (that's not us!) should view this job as no more dreadful than re-bedding the deck hardware. But along with bulkhead re-tabbing, the best peace of mind you can buy.

Think I'll call my welder today. There's a brief window here where I have both spare money and spare time -- a condition practically unheard of. :lol:

Thanks for pioneering the use of the endoscope & for the info on the bilge/bulkhead gap, Mark. I knew it was there (cuz the previous owners had pumped some squirt foam in!) but never suspected you could get a camera shot thru it. And the endoscope image really isn't bad quality at all (once I figured out which way is up). Non-destructive inspections will encourage more inspections!
Liese
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Joined: Wed Oct 31, 2012 8:08 am

Re: Mast step truss, inevitably:(

Post by Liese »

Wow,
Really good Job!
I think I will publish an official Link on ballad.de.
Unfortunately on our ballad the gap is closed (#2007) - but I keep the endoscope in mind.

Best regards,
Jörg
Ballad.de
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